So, i've made my own "Parthurnax" duplicate changed his name, added the template for "Sit" so he is sitting in the map when you meet him right now, i clicked the FLAGS tab and ticked the following boxes:
. MUST COMPLETE
. ALLOW IDLE pvssyR
. IGNORE COMBAT
. HELLOS TO PLAYER

now in game you'd think that he would have a talk option with all the voice dialog chat that he normally has from the "Throat Of The World" but there's no talk option and niether does he chat at all...

what must i do to make it so???

Check his voice options.

I've made both dragons and giants talk, just by giving them a voice (iirc). Look at that area anyway.

Parth's Dialog is most likely almost absolutely 100% Conditioned for the Unique Reference of Path to use it -> if you want your Parth Copy to be able to use it you are going to have to give your Copy Parths Voice Type (if it already does not have it) and then your going to have to find every Parth Dialog Entry and Condition your Dragon to use it as well.

In a similar vein, I'm trying to get some dragons I've made to taunt/have a death quote like the very first dragon you come across in the game. I've written stuff but it lasts on screen for a split second. Is it because I have no sound to go along with it? I've tried duplicating what how his dialogue works (in MQ104 I think) but I might've done something wrong.

Yes.